Description of the Prior Art
Electromagnetic radiation detectors are used for detecting electromagnetic radiation of various wavelengths, such as infrared radiation, emitted from missiles, aircraft or other objects. For example, infrared detectors are used on guided missiles to direct them to infrared radiation emitting targets.
One such infrared detector is disclosed in: "A New Thermal Radiation Detector Using Optical Heterodyne Detection Of Absorbed Energy", by Christopher C. Davis and Samuel J. Petuchowski, Infrared Physics, Volume 23, No. 4, pages 177-182 (1983). The operation of this detector is based upon a phase fluctuation optical heterodyne (PFLOH) technique described in an article entitled: "Phase Fluctuation Optical Heterodyne Spectroscopy Of Gases", by Christopher C. Davis and Samuel J. Petuchowski, Applied Optics, Volume 20, No. 4, pages 2539-2554 (1981).
The detector described in the article utilizes optical heterodyne techniques to detect a small density change which occurs in a gas when it is heated in a spatially non-uniform manner by electromagnetic radiation. The heating can occur in one of two ways. Either the gas itself absorbs the input radiation to be detected directly or indirectly by heat by thermal conduction from an absorbing surface or membrane.
When a thin membrane is used, the infrared signal irradiates the membrane causing the membrane to heat up. Since the membrane is disposed relatively close to one arm of an interferometer, the membrane exchanges heat with a gas disposed in that arm of the interferometer which, in turn, causes the density of the gas close to the membrane to decrease. This decrease in the density of the gas results in a decrease in the index of refraction. The change in the index of refraction is detected by an interferometer.
Modulated radiation used to heat the gas results in density modulation of the gas which, in turn, phase modulates the single frequency laser beam passing through the gas. The resulting modulated laser beam results in the modulation frequency of the input radiation. This modulated beam is demodulated at a photodetector at the interferometer output.
Referring to FIG. 1, a reference beam A and a probe beam B are derived from the single frequency laser beam by way of a beam splitter. Reference beam A serves as a local oscillator beam and is disposed sufficiently far from the absorbing surface such that the thermal conduction only has a negligible effect on it. Probe beam B passes very close to the absorbing surface which is irradiated by a modulated input source of radiation. This irradiation leads to a temperature modulation of the gas along the path of the beam. This, in turn, results in a refractive index modulation of the gas along the path of the beam B and consequently phase modulation of the beam B. The modulated beam B is demodulated by a photodetector to provide a signal representative of the modulation frequency of the input radiation.
The radiation detectors described above are only able to quantitatively measure the amount of radiation falling upon them. This type of detector is unable to produce a two-dimensional picture of the radiation-emitting object. This device relies upon the transport of heat from the illuminated membrane to the adjacent gaseous medium.